The present invention relates to a process for screening features, such as wiring lines and vias on an electronic substrate and, more particularly, relates to a process for screening such features using a low viscosity paste.
Multilayer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, an MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have via holes, more typically called vias. Prior to lamination, the vias are typically filled with an electrically conductive paste, such as a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, conventional ceramic structures are formed from ceramic greensheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After via formation, metal paste screening, stacking and laminating, the greensheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the metal paste screening step of this process.
The patterning of ceramic greensheets used for the production of MLC substrates typically involves the silk screening or extrusion screening of a metal-filled organic paste. In order to achieve the necessary dimensional control of the metal circuitry features, such as wiring lines and vias, the rheology of the metal paste must be carefully controlled such that the paste does not bleed out once applied to the ceramic greensheet. To achieve this type of behavior, the metal pastes typically have a relatively high viscosity which is not conducive to filling the vias. For via filling, the rheological requirements for the paste are typically quite different compared to the paste requirements for the wiring line patterns. For this reason, most MLC processes involve two step screening where the wiring line pattern is formed in one process and the vias are filled in another process step.
Hoebener et al. U.S. Pat. No. 5,492,266, the disclosure of which is incorporated by reference herein, recognizes the difficulty of screening solder in fine pitch patterns due to, among other things, solder paste viscosity and rheology. Hoebener et al. resolve their problem by screening solder paste through a stencil and then reflowing the solder before the stencil is removed.
Booth et al. U.S. Pat. No. 5,543,585, the disclosure of which is incorporated by reference herein, discloses the screening of a conductive adhesive emulsion (consisting of conductive adhesive and a solvent) into holes for electrically connecting a chip and a substrate. After application of the conductive adhesive, the conductive adhesive is heated to drive off the solvent.
Capote et al. U.S. Pat. No. 5,376,403, the disclosure of which is incorporated by reference herein, discloses an electrically conductive composition which may include a metal powder, a solder powder (in addition to the metal powder), a cross-linking agent, a resin and a reactive monomer. The electrically conductive composition may be applied as a wiring pattern by screening. After application, the electrically conductive composition is heated to cure it. The electrically conductive composition may be used on any substrate that can withstand the curing temperature.
Ideally, one metal-filled paste would be available which can be used for both via filling and wiring line patterning of the ceramic greensheet. To achieve this objective, the paste must have a low enough viscosity to fill the small vias yet be able to produce fine wiring lines and patterns of the wiring circuitry on the surface of the ceramic greensheet. Further, the paste must xe2x80x9cset upxe2x80x9d, i.e., increase its viscosity, without excessive bleeding into the ceramic greensheet and cannot, due to the nature of the ceramic greensheet, be cured by heating.
Accordingly, it is a purpose of the present invention to have a metal-filled paste which is suitable for producing via filling and wiring line patterning of ceramic greensheets.
It is another purpose of the present invention to have a metal-filled paste which has a low enough viscosity to fill the vias and form wiring lines and patterns.
It is yet another purpose of the present invention for the metal-filled paste to set up after application of the metal-filled paste to the ceramic greensheet.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
A first aspect of the present invention relates to a process for screening features on an electronic substrate, the process comprising the steps of:
obtaining a low viscosity paste comprising at least a metallic material, solvent and binder, said low viscosity paste having an excess of solvent over what is needed to dissolve the binder;
screening the low viscosity paste onto an electronic substrate to form at least one feature;
blotting the excess solvent with a blotting material so as to increase the viscosity of the low viscosity paste; and
removing the blotting material.
A second aspect of the invention relates to a process for screening features on an electronic substrate, the process comprising the steps of:
obtaining a low viscosity paste comprising at least a metallic material and one of the following: solvent/binder/reactive monomer/cross-linking agent or resin/cross-linking agent; and
screening the low viscosity paste onto an electronic substrate to form at least one feature wherein the reactive monomer/cross-linking agent or resin/cross-linking agent cause an increase in the viscosity of the low viscosity paste.
A third aspect of the present invention relates to a process for screening features on an electronic substrate, the process comprising the steps of:
obtaining a low viscosity paste comprising at least a metallic material, a solvent, a binder, a reactive monomer, and a cross-linking agent; and
screening the low viscosity paste onto an electronic substrate to from at least one feature wherein the reactive monomer and cross-linking agent cause an increase in the viscosity of the low viscosity paste.
A fourth aspect of the present invention relates to a process for screening features on an electronic substrate, the process comprising the steps of:
obtaining a low viscosity paste comprising at least a metallic material, a resin and a cross-linking agent; and
screening the low viscosity paste onto an electronic substrate to from at least one feature wherein the resin and cross-linking agent cause an increase in the viscosity of the low viscosity paste.
A fifth aspect of the present invention relates to a process for screening features on an electronic substrate, the process comprising the steps of:
obtaining a low viscosity paste comprising at least a metallic material and one of the following: solvent/binder/reactive monomer or resin;
applying a cross-linking agent to a surface of an electronic substrate; and
screening the low viscosity paste onto an electronic substrate and the cross-linking agent to form at least one feature wherein the reactive monomer/cross-linking agent or resin/cross-linking agent cause an increase in the viscosity of the low viscosity paste.